As development of light-weighted construction and high performance for automotive vehicles, etc. has progressed, high stress design has been required for valve springs, clutch springs, brake springs or the like. Springs excellent in fatigue strength and sag resistance have been demanded. In particular, there is a strong demand for high stress design of valve springs.
Recently, it has been a custom that valve springs are primarily produced by cold coiling an oil-tempered wire that has been applied with quenching and tempering. According to the Japanese Industry Standards (JIS), for example, an oil-tempered wire (according to JIS G3561) for valve springs is separately defined from an ordinary oil-tempered wire (according to JIS G3560). Thus, it is required to strictly control the kind of steel, allowable impurity content, depth of flaw, etc.
The oil-tempered wire has the following advantage and disadvantage. As regards the advantage, since the oil-tempered wire has tempered martensite, it can provide springs of high strength, and it has excellent fatigue strength and sag resistance. As regards the disadvantage, there is required a large-scaled facility and cost for heat treatment such as quenching and tempering to produce the oil-tempered wire.
Some of valve springs of low load stress are obtained by drawing carbon steel that has ferrite/pearlite or pearlite to increase strength (also called “hard-drawn wire”), and by cold coiling the hard-drawn wire. According to the JIS, such a wire belongs to the criteria of “Piano Wire Type V” for “valve springs or like springs” in the criteria of piano wires according to JIS G3522.
Springs made of the aforementioned hard-drawn wire (hereinafter, referred to as “hard-drawn springs”) can be produced with a low cost because heat treatment is not required in the production method. However, the wire in which ferrite/pearlite or pearlite has been subjected to drawing has low fatigue properties and low sag resistance. Accordingly, even if such a wire is used for springs, it cannot provide for high-strength springs that are required in the recent technology.
There also have been studied various techniques to produce high-strength hard-drawn springs in light of the advantage of low-cost production. Japanese Unexamined Patent Publication No. HEI 11-199981 proposes an exemplified method for obtaining cementite of a specific configuration by performing a wire drawing process to pearlite in eutectoid-hypereutectoid steel, which is usable as a piano wire having properties equivalent to an austempered wire. This method, however, unavoidably raises the production cost because the production process is complicated such that a step of changing the wire drawing direction is additionally required.
In view of the above, an object of this invention is to provide a steel wire rod used for producing hard-drawn springs capable of exhibiting fatigue strength and sag resistance equivalent to or higher than springs produced by an oil-tempered wire, a wire for hard-drawn springs, such hard-drawn springs, and a useful method for producing such hard-drawn springs with a low cost.